The invention relates generally to marine seismic prospecting and in particular to apparatus and methods for reducing the effects of undesired seismic reflections and noise in sensors towed behind a survey vessel, in sensors laid on the sea bottom, or in sensors in autonomous nodes.
In towed marine seismic exploration, a hydrophone array is towed behind a marine vessel 20 near the sea surface 22, as in FIG. 1. The hydrophones are mounted in multiple sensor cables commonly referred to as streamers 24. The streamers serve as platforms for the hydrophones. A seismic sound source 26, also towed near the sea surface, periodically emits acoustic energy. This acoustic energy travels downward through the sea, reflects off underlying structures or subsea strata 28, and returns upward through the sea to the hydrophone array. Reflected seismic energy arrives at towed-array receive points. The hydrophone array contains many such receive points and records the upward traveling seismic acoustic wavelet from the seabed 30 at each of the receive points. The hydrophone recordings are later processed into seismic images of the underlying structures.
Noise is a major consideration in towed streamer operations. Noise sources include swell noise and wave noise from the sea surface. And towing the streamer through the water causes noise. Some of this noise propagates through the streamer and some through the water column itself. The typical way of dealing with noise sources is to use a combination of temporal and spatial filtering. Temporal filtering is accomplished by discrete digital sampling of the hydrophone signals in time with weighting applied to the samples. The hydrophone channels also include analog filters to prevent aliasing of signals at frequencies greater than half the sample rate. The spatial samples are typically formed by group-summing individual hydrophone outputs so that pressure noise propagating along the length of the streamer is attenuated. This spatial sampling has no impact on noise that propagates in a direction orthogonal to the streamer axis. Typical hydrophone groups consist of eight or so hydrophones in a 12 m section of the streamer.
Acoustic impedance, ρc, is the product of the density and the speed of sound in a medium. Reflection of at least some of the sound-wave energy occurs whenever a change in acoustic impedance is encountered by the sound waves. The energy that is not reflected is transmitted (refracted) beyond the boundary between the two regions of different acoustic impedances. The pressure waves are compression waves, which induce particle motion in the direction of propagation. At a planar interface between two different homogenous media, a sound wave reflects at an angle equal to the angle of incidence θ1 and refracts at an angle θ2. The refraction angle is given by:θ2=sin−1(c2 sin θ1/c1).The subscript refers to the sound wave moving from medium 1 to medium 2 and c1 and c2 are the speeds of sound in each medium. If the incident angle θ1 is zero, then the refracted energy propagation path will be at an angle θ2 of zero.
For an incident angle θ1 of zero and no energy converted to shear energy, the reflection coefficient at the water-air interface is described by:
                              R          pp                =                ⁢                                                            ρ                2                            ⁢                              c                2                                      -                                          ρ                1                            ⁢                              c                1                                                                                        ρ                2                            ⁢                              c                2                                      +                                          ρ                1                            ⁢                              c                1                                                                            ≈                ⁢                  -          1.                    The reflected energy at the water-air interface is R2pp, or nearly 1, making the sea surface a near perfect reflector of sound energy. After returning from the sea bottom or the target of interest, the energy is again reflected by the sea surface back to the streamer. Because a typical hydrophone has an omni-directional response, the hydrophone array also records a ghost response, which is the seismic acoustic wavelet reflected from the sea surface and arriving delayed in time and reversed in polarity. The ghost is a downward-traveling seismic acoustic wave that, when added to the desired wave, detracts from the recorded seismic image. The ghost-causing reflection can also continue to the sea bottom or other strong reflector and be reflected back up to again interfere with the desired reflections and further degrade the image. These reflections are commonly referred to as multiples.
For a vertically traveling pressure wave, the ghost produces a notch in the frequency spectrum of a hydrophone response at fnotch=c/2d, where c is the speed of sound and d is the streamer depth. Seismic streamers have been conventionally towed at a depth of 10 m or less. At a depth of 10 m, the notch frequency (fnotch) is 75 Hz. A frequency response extending beyond 100 Hz is required for high seismic image resolution. Because the notch frequency is inversely proportional to the tow depth, streamers are often towed at shallower depths to improve the resolution of a seismic image. Towing at shallow depths is problematic because noise from the sea surface begins to interfere with the desired seismic signals. These effects are worsened as weather deteriorates, sometimes causing the crew to discontinue operations until the weather improves. The elimination of ghost-notch effects would enable towing at greater depths farther away from surface disturbances.
Ocean bottom systems, in which the seismic sensors are placed on the seabed, reject ghosts and multiples by a technique commonly known as p-z summation. In an acoustic wave, the pressure p is a scalar, and the particle velocity u is a vector. A hydrophone, with a positive omni-directional response, records the seismic acoustic wave pressure p. A vertically oriented geophone or accelerometer records the vertical component of the seismic acoustic-wave particle velocity uz, with a positive response to up-going signals and a negative response to down-going signals. In p-z summation, the velocity signal is scaled by the acoustic impedance ρc of seawater before it is added to the pressure signal. A gimbaled single-axis sensor is also scaled to account for the change in sensitivity of the particle-motion sensor due to the off-axis arrival of any received signals. If an accelerometer is used, its output signal can be integrated to obtain the velocity signal, or the hydrophone signal can be differentiated so that it can better spectrally match the accelerometer. This produces a compound sensor that has a full response to the upward traveling wave and at least a partially muted response to the downward traveling wave to reject the ghost and multiples. One such method of signal conditioning and combination of signals to get a single de-ghosted trace is described in U.S. Pat. No. 6,539,308 to Monk et al. FIG. 2 is a two-dimensional (2D) representation of the response of a particle-velocity sensor. FIG. 3 is a 2D representation of the response of an omni-directional hydrophone summed with the response of a vertical particle-motion sensor. The full three-dimensional responses can be envisioned by rotating the 2D responses about their vertical axes.
Recently there has been interest in using techniques similar to p-z summation in towed-streamer acquisition to allow deeper tows without interference from ghost-notch reflections. Operating a particle-motion sensor in a seismic streamer presents a problem because the streamer experiences accelerations due to towing or sea surface effects that are large compared to accelerations caused by the desired seismic reflections. Moreover, these unwanted accelerations are in the same spectral band as the desired reflection response. When a towing vessel encounters ocean waves, there are small perturbations in the speed of the vessel. The vessel also typically undergoes a yawing motion. FIG. 4 depicts energy being imparted to the streamers 24 by speed variations 32 and yawing motion 34. FIG. 5 is a side view depicting energy causing accelerations and transverse waves in the streamer 24. (The energy's effect on the streamer is exaggerated in FIG. 5 for illustrative purposes.) Most of the energy is attenuated by elastic stretch members 36, typically in front of the sensing arrays. While the energy is greatly attenuated, some does remain. Accelerations a caused by planar pressure waves due to the desired seismic reflections are given by:
      a    =                  p        ⁢                                  ⁢        2        ⁢        π        ⁢                                  ⁢        f            Z        ,where p=the acoustic sound pressure level, f is the frequency, and Z is the acoustic impedance. Performance of a particle-velocity measuring system should be near the ambient noise limits. Typically, seismic-data customers require ambient noise from streamer hydrophone systems to be below 3 μbar. Since the acoustic impedance of seawater is 1.5 MPa·s/m, a 3 μbar pressure wave at 4 Hz produces particle accelerations of roughly 0.5 μg. FIG. 6 shows a mechanical model of the frequency response of typical cable axial accelerations in the middle of a streamer. The presence of a secondary peak at 4 Hz, only 1.5 orders of magnitude below the primary peak, indicates that, in some cases, the cable dynamic motion can be greater than the seismic signal to be measured.
U.S. Pat. No. 7,167,413 to Rouquette uses an accelerometer in a seismic streamer to reject the ghost-notch effect. Rouquette uses a mass-spring system to reduce the effect of cable dynamics on the accelerometer and a load-cell system to measure and reject the cable-motion-induced noise on the accelerometer. The Rouquette system relies on well-known complex mechanical relationships that do not remain constant with manufacturing tolerances, aging, and environmental conditions. Rouquette uses a signal-processing adaptive algorithm to derive the relationship of the load-cell-sensor-and-mass-spring system to the acceleration acting on the accelerometer in situ. Rouquette describes a complex mechanical and electronic system.
U.S. Pat. No. 7,239,577 to Tenghamn et al. describes an apparatus and method for rejecting the ghost notch using an acoustic-wave particle-velocity sensor. Tenghamn et al. teaches the use of a fluid-damped, gimbaled geophone. It is known in the art that the fluid encapsulating the geophone is chosen to provide damping of the sensor swinging on its gimbals. While not described in Tenghamn et al., it is known in the art that a mass-spring vibration-isolation system can reduce the effect of cable mechanical motion on the geophone response. Motion of the geophone caused by cable mechanical motion is indistinguishable from acoustic-wave particle motion in the geophone response. The desired seismic-wave particle motion is obscured by cable mechanical motion in Tenghamn et al. This technique also gives the response similar to the cardioid in FIG. 3, where there are still undesired signals coming from the surface and being induced by streamer excitation along the streamer axis.
U.S. Pat. No. 7,359,283 to Vaage et al. describes a method of combining pressure sensors and particle-motion sensors to address the impact of mechanical motion on the particle-motion sensors. In this method, the response of the particle-motion sensor below a certain frequency f0 is not used, but only estimated from the pressure-sensor response and the known pressure-sensor depth. The frequencies rejected are those for which mechanical motion of the streamer is expected. The estimated response has poor signal-to-noise ratio at the lower frequencies of interest. This rejection below a certain frequency is not optimal as it also rejects valid signals in an important low-frequency band where deep-target data is likely to exist.
While these patents all describe methods to reject the ghost notch in a seismic streamer, none adequately addresses the effects of streamer tow and other noise that affects the particle-motion sensor or hydrophone measurements. All also fall short of producing high-fidelity, sensed acoustic-wave components with good signal-to-noise ratio down to the lowest frequencies of interest.